It is easy to see why some viewers might worry about Secret Life. Channel 4’s new drama portrays a convicted paedophile struggling not to reoffend after his release into the community. The central, unavoidable issue in the script by writer-director Rowan Joffe is that it presents the paedophile Charlie Webb (Matthew Macfadyen) as a sympathetic, and often extremely likeable character. This could easily seem like an attempt to arouse sympathy for the worst kind of sex offender and an example of Channel 4 at its trendy, liberal worst. To some, it might be more evidence of the channel’s lack of moral compass, an extension of its preoccupation with deviant sex into a wholly inappropriate area.

Yet such a reaction really would miss the point. Macfadyen’s character is portrayed as a “nice” man, says Joffe, because most paedophiles are not the shabby, gap-toothed kidnappers of parental nightmare. He quotes Detective Chief Inspector Bob Maclachlan, a former head of Scotland Yard’s paedophile unit, who says that many offenders seem to be intelligent and charming men. “Monsters don’t get your kids. Nice men do.”

In an early scene, Charlie, a former swimming coach, is baldly confronted with the fact that he has raped three girls between the ages of 8 and 12, “vaginally, orally and anally”. For Macfadyen, who has three young children with his wife Keeley Hawes, the character’s charm was much the most disturbing aspect of the role. Best known as the ultra-cool spy Tom Quinn in Spooksand as a smouldering Mr Darcy in the 2005 film of Pride and Prejudice, Macfadyen was cast in the role, says Joffe, because of his complexity.

“When I was reading the script I was always pulling myself up short,” he says. “I’d find it very sympathetic and then I’d find it repellent.” A sequence at a fairground was particularly challenging, he tells me. The first few scenes, when Charlie approaches two young girls, were improvised. “I just had a bash and hoped for the best.”

Later Charlie attracts a 12-year-old called Michaela. “He goes into predator mode, in the same way he would insinuate himself into a relationship with girls he was coaching. He would have tried to be quite young and cool, which is so awful. Rowan was talking about the way many abusers get stuck in the emotional age when they were abused themselves, so that was in the mix somewhere too.”

Viewers may also find themselves led into disturbing territory by the way the film allows us to empathise with Charlie. Wanting him to escape when he is chased by thugs with baseball bats is one thing; watching the apparent kindness and charm with which he begins to “groom” Michaela is another altogether.

Most screen dramas about child abuse have focused on sinister priests or bullying adults running care homes. This story is about a far more common form of paedophilia, which confuses parental, or quasi-parental love with sexual love.

Yet the essence of the story is Charlie’s struggle not to reoffend. He has been given strict behavioural guide-lines, which he observes meticulously. He wears a rubber band around his wrist which he thwacks every time he feels an improper urge, to break his chain of thought. Wherever possible he avoids children.

Joffe says the drama was inspired five years ago when he read about the closure of Wolvercote, Britain’s last paedophile rehabilitation centre in Surrey. Since then all efforts to open such centres have foundered because of local objections.

Yet Wolvercote provided offenders with the mix of tough, close monitoring and emotional support they badly need. As Donald Findlater, its former director, says, it had a proven track record in cutting reoffending. In other words the centre saved some children from abuse. When Charlie’s centre closes, he finds himself isolated, alienated and persecuted, and his resolve begins to crack.

Joffe is the first to admit that this is an extremely difficult issue. When asked how he’d feel if such a centre opened near his home, he says that he can’t honestly say.

Whatever the likely controversy, the play has won support from two charities. Findlater, the man behind Stop It Now!, a campaign to spot signs of paedophile behaviour and challenge them, was “delighted the film had been made”, while Peter Saunders, of the National Association for People Abused in Childhood, calls it “an important film which presents society with a stark choice”.

Macfadyen is wary of media outcries on the issue. “People imagine that you’re defending paedophiles by saying let’s have a reasoned debate about it,” says Macfadyen, “but why are we so scared? The hysteria and shrillness come from a kind of fear, and apparently it’s peculiar to Britain and the States. On the Continent it’s much more reasonable.”

Secret Life, Thur, C4, 9pm

Editor's choice

• U2 at Nou Camp Stadium, Barcelona
• The 50 Best Movie Villains
• The 15 golden rules of theatre etiquette